1. Field of Technology
This invention relates to a temperature measurement apparatus using a temperature sensitive device such as a thermistor, or a temperature measurement resistor. The invention is aimed at suppressing variations or errors of measured temperature values arising from temperature variation surrounding the circuit devices of the termperature measurement apparatus and power supply voltage variation.
2. Prior Arts
FIG. 1 shows one example of conventional temperature measurement apparatus, which comprises a bridge circuit 1 including a temperature sensitive device, a power source 2, an amplifier 3 and a display apparatus 4 of measuring temperature values such as a meter or a numeral display apparatus. It is necessary for the temperature measurement apparatus to select resistor devices having preferably small temperature coefficient except the termperature sensitive device in the bridge circuit 1 and it is also necessary to constitute the temperature measurement apparatus in such a manner that the power source 2 and the amplifier 3 are less influenced by temperature variation. There are several ways to countermeasure the above intervention for the temperature measurement (i) the power source 2 and the amplifier 3 are specially devised in their circuit configuration, (ii) circuit elements are selected from such transistors, ICs, resistors and capacitors, etc. that have favourable temperature characteristics, and (iii) circuits and devices are suitably used to calibrate and/or compensate the temperature variation. The abovementioned countermeasures give rise to make the temperature measurement apparatus cost much.
Another example of a conventional temperature measurement apparatus is shown in FIG. 2. The apparatus comprises an oscillator circuit 5 which includes a time constant circuit 6 consisting of at least a resistor and a capacitor and determining an oscillation frequency or a width of oscillation pulses. In the circuit 6, one resistor is a temperature sensitive device used for a temperature measurement probe. The block 7 designates the remaining part of the oscillator circuit 5 excluding the time constant circuit 6. The apparatus also comprises a reference frequency oscillator 8 which is usually an oscillator making use of a crystal oscillator and is for measurement of the frequency of the oscillator circuit 5 or the width of the oscillation pulses thereof, a frequency counter 9 which counts number of output pulses of the oscillator circuit 5 during one cycle of output pulses of the reference frequency oscillator 8, a conversion circuit 10 which converts a counted number of the frequency counter 9 into a value scaled in temperature, and a display apparatus 11 for displaying a temperature value.
In the temperature measurement apparatus of FIG. 2, temperature is measured by measuring variation of the oscillator frequency of the oscillator circuit 5 by utilizing a certain reference frequency. The oscillation frequency of the oscillator circuit 5 varies when temperature around the temperature sensitive device varies and this frequency variation is detected with respect to the reference frequency. To diminish an error-inducing effect on the electric parts due to variation of the environmental temperature around the temperature measurement apparatus, it is necessary that the oscillation frequency of the oscillator circuit 5 is not affected by the environmental temperature and besides that the reference frequency of the reference oscillator 8 is stable with respect to the variation of the environmental temperature. This means that temperature dependency of the utilized devices except the temperature sensitive device in the oscillator circuit 5 must be small and the circuit configuration must be devised to achieve such purpose. This requirement leads to the use of a crystal oscillator, whose temperature coefficient of the oscillation frequency is extremely small, for the reference oscillator 8.
There is also a possibility that the temperature measurement is accompanied with errors arising from the variation of the power source voltage. It is not so significantly problematic for the stabilization of the power source voltage when commercial electricity is available, although it costs much to produce a stabilized voltage. This contrasts to a case where a battery power source is used thereby necessitating batteries of high voltage in order to stabilize the voltage thereby, and where power consumption increases for the voltage stabilization causing shortened battery life.